Impulsivity toward food reward is related to BMI: Evidence from intertemporal choice in obese and normal-weight individuals
Introduction
The daily consumption of excessive quantity of food is typical of obesity and binge eating disorder (BED). Central to these pathologies is the exaggerate saliency of food reward at the expense of other rewards (Volkow, Wang, Fowler, & Telang, 2008). This results in increased engagement in impulsive behavior and decision toward immediate food outcome (Davis et al., 2004, Volkow and Baler, 2015). During intertemporal choice (i.e., decision between smaller-immediate outcome and larger-delayed outcome) the ability to postpone immediate gratification is crucial to achieve healthy long-term goals, including weight control and the cessation of drug misuse (Sellitto, Ciaramelli, & di Pellegrino, 2011). Humans typically devalue rewards as a function of time availability, a phenomenon known as temporal discounting (TD) of future rewards (Ainslie, 1974). Namely, the subjective value of a reward is greater when delivered immediately (e.g., the gratification coming from eating a chocolate praline) than when delivered in the future (e.g., losing weight by being on a diet for one month) (Cardinal et al., 2001, Green and Myerson, 2004, Kalenscher et al., 2005, Myerson and Green, 1995). The less individuals are able to override tempting immediate reward in favor of long-term gain, the more they are considered impulsive (Frederick et al., 2002, Sellitto et al., 2011).
Several studies showed that obese individuals have increased preference for smaller-immediate gratification over larger-delayed reward as compared with healthy controls. Specifically, higher discount rate for hypothetical future money has been found in obese women compared to normal-weight women (Davis et al., 2010, Davis et al., 2004, Weller et al., 2008), and in obese smokers adolescents compared to their normal-weight peers (Field, Santarcangelo, Sumnall, Goudie, & Cole, 2006). In addition, impulsivity toward immediate monetary reward predicted the amount of caloric intake in obese individuals (Appelhans, Pagotom, Schneider, Whited, & Liebman, 2011).
Impulsivity toward immediate reward has been recently associated with different psychopathological (i.e., addiction and anxiety), and neuropsychological conditions (i.e., executive disfunctioning), as well as personality traits (i.e., reward dependence and harm avoidance) (Bickel et al., 2012, Huckans et al., 2010, Jenks and Lawyer, 2015, MacKillop et al., 2011, Malesza and Ostaszewski, 2013, Rounds et al., 2007, Sellitto et al., 2011, Story et al., 2014). All these factors may lead to misinterpretation of findings related to TD when the effect of obesity and BMI is assessed. For instance, depression has been suggested to be a moderator of impulsivity toward immediate palatable food reward in overweight/obese individuals (Privitera, McGrath, Windus, & Doraiswamy, 2015), thereby saliency attribution to food might be modulated by psychopathological factors. However, most obese people do not show clear psychopathological condition related to behavioral disinhibition, such as binge eating disorder or depression, and obesity is not classified as a mental disorder (Berkowitz and Fabricatore, 2011, Carpiniello et al., 2009, Fabricatore et al., 2005, Malik et al., 2014).
An alternative explanation may lay in the repeated stimulation of the reward system induced by prolonged exposure to the rewarding stimulus (i.e., food), which would lead to maladaptive stimulus-reward associations. This, in turn, would contribute to an enhancement of the incentive value of a reward at the expense of others (Berridge & Robinson, 1998). Thus, despite the absence of traits of impulsivity and alterations in response inhibition and cognitive control, an exaggerate saliency of food reward, at the expense of other rewards, could be hypothesized in obese individuals without binge eating. This may results in increased engagement in impulsive behavior and decision specifically toward immediate food outcome in this population (Davis et al., 2004).
It has been also suggested that the preference for smaller but immediate rewards could be related to body mass index (BMI): The higher the BMI, the higher the degree of impulsivity toward immediate rewards (e.g., Ikeda, Kang, & Ohtake, 2010). However, most of TD research in humans focused on monetary reward (Critchfield and Kollins, 2001, Reynolds, 2006), and, to our knowledge, only two studies assessed discounting behaviour for both food and monetary rewards in participants with BMI raging from normal-weight to obesity (Manwaring et al., 2011, Rasmussen et al., 2010). In one (Rasmussen et al., 2010), BMI was found not to be a predictor of discounting behaviour for neither food nor monetary reward. In the other (Manwaring et al., 2011), obese individuals with BED, characterized by compulsive eating, were more sensitive to immediate gratifications independently of reward type, whereas obese individuals without BED selectively showed greater sensitivity to immediate food reward as compared to normal-weight individuals. However, this latter study did not explore the relationship between the degree of discounting of future food reward and BMI.
In the present study, we tried to reconcile previous discrepancies by investigating intertemporal choice for both primary (i.e., food) and secondary (i.e., money and discount voucher) reward in obese individuals and normal-weight controls in a more systematic manner. Participants made a series of hypothetical decisions between smaller-immediate amounts of rewards and larger-delayed amounts of rewards, separately for each commodity. Furthermore, to avoid the possibility that differences between obese and normal-weight individuals in the TD tasks might depend on more general differences in neuropsychological and psychological factors related to impulsivity, we submitted participants to several tests. Neuropsychological evaluation focused on the assessment of major executive functions, frequently reported to be altered in obese compared to normal weight individuals (Prickett, Brennan, & Stolwyk, 2015). Thus, a series of paper and pencil as well as computerized measures were adopted to investigate different executive domains, as for instance cognitive control (through the Simon task), attention and switching ability (through the Trial Making Test A, B), and working memory (through the Sternberg test).
Moreover, subjects filled out questionnaires assessing impulsivity, personality traits, and the presence of psychopathological state at the time of the study, which are variables that may be linked to discounting behavior (e.g., Ostaszewski, 1998). This, in order to: (i) replicate previous findings of increased discounting for future reward in obese population; (ii) to investigate whether obese people’s impulsivity is selective for food, independently from others psychopathological/neuropsychological factors. Thus, in line with the incentive sensitization theory, we hypothesized that even in absence of impulsivity traits, binge eating disorder, or other psychopathological and neuropsychological condition related to behavioral disregulation, obese individuals would be more impulsive than normal-weight controls when faced with food stimuli compared with money and discount voucher, and that this tendency would be positively related to BMI.
Section snippets
Participants
Twenty-three obese subjects and 23 normal-weight subjects matched on age and education were enrolled in the study (Table 1). Inclusion criteria consisted of BMI > 30 kg/m2 (obese) or BMI ranging between 19 and 26 kg/m2 (normal-weight controls), and age ranging between 18 and 50 years (see Table 1 for demographic details). Exclusion criteria for all participants were the presence of clinical history of eating and/or other neurological/psychiatric disorders, history of alcohol and/or drug abuse,
TD task
Fig. 2A shows that the hyperbolic discounting curve for food of obese individuals is significantly steeper than that of normal-weight individuals. Conversely, discounting curves for money and discount voucher did not differ between the two groups (Fig. 2B and C). Please, note that the k value for each curve reflects the geometric mean of the group, which corresponds to mean of the log-transformed values.
The above impressions were confirmed by statistical analyses. Since the hyperbolic k
Discussion
The present study investigated hypothetical intertemporal choice for primary and secondary reward in obese and normal-weight individuals.
Sensitivity to immediate reward was measured using three TD tasks in which three different types of reward were used (i.e., food, money, discount voucher), separately. Results revealed that obese individuals were more prone to choose immediate smaller reward (i.e., increased discounting of future reward) compared to normal-weight controls, and that this
Acknowledgments
The authors thank Debora Vego Scocco, Maria De Salvo and Gina Boffo for helping during participants’ recruitment and data collection. S.S. and G.D. designed the study. S.S. and T.N. collected the data. S.S., M.S., G.D. and G.T. wrote the paper. All authors read and approved the paper. The study was carried on under the auspices of CIRMANMEC – University of Padova, Italy. The authors have no competing interests.
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